More than 1 million people have fled their homes since fighting erupted in the world's youngest country in December

By Michelle Nichols

UNITED NATIONS, April 23 (Reuters) - U.N. Security Council members are considering sanctions on South Sudan's warring parties, envoys said on Wednesday, after U.N. peacekeeping chief Herve Ladsous demanded "serious consequences" be imposed to force an end to the violence.

Ladsous and U.N. Assistant Secretary-General for human rights Ivan Simonovic briefed the 15-member council on a recent escalation in attacks on civilians, including an ethnic massacre in the oil town of Bentiu and the killing of dozens of people who had sought refuge inside a U.N. peacekeeping base in Bor.

"Unless there are serious consequences for the parties to cease the violence and engage in meaningful talks ... the toll on innocent civilians will continue to rise," Ladsous told reporters after the closed-door council meeting.

"The United Nations is doing everything it can to protect the civilians that are fleeing the violence, the war, but let us never forget that the primary responsibility for protection of civilians is with the government," he said.

Nigerian U.N. Ambassador Joy Ogwu, president of the council for April, said there was a lot of support among council members for pursuing sanctions on South Sudan.

"I think we are ready to go down the road of sanctions," French U.N. Ambassador Gerard Araud told reporters.

U.S. Ambassador to the United Nations, Samantha Power, posted on Twitter after the briefing: "For the sake of the people of South Sudan, international community must sanction political spoilers and those who target civilians."

The United States and the European Union have already threatened South Sudan with sanctions. President Barack Obama earlier this month authorized possible targeted sanctions against those committing human rights abuses in South Sudan or undermining democracy and obstructing the peace process.

U.N. 'SOUL-SEARCHING'

More than 1 million people have fled their homes since fighting erupted in the world's youngest country in December between troops backing President Salva Kiir and soldiers loyal to his sacked deputy, Riek Machar.

The fighting has exacerbated ethnic tensions between Kiir's Dinka people and Machar's Nuer. Negotiations between the Kiir government and rebels loyal to Machar have failed to advance since the Jan. 23 signing of a ceasefire which never took hold.

The United Nations accused the rebels of hunting down men, women and children last week in a hospital, church and mosque in the capital of the oil-producing Unity state and then killing them based on ethnicity and nationality.

After the rebels seized Bentiu, Dinka residents of Bor town in Jonglei state attacked a U.N. base on Thursday where about 5,000 people, mostly Nuer, were sheltering.

They pretended to be peaceful protesters delivering a petition to the United Nations before opening fire on the base, killing some 58 people and wounding 98, including two Indian peacekeepers, the United Nations said.

Thousands of people have been killed and tens of thousands have sought refuge at U.N. bases around South Sudan after the violence spread across the country.

The Security Council is due to renew the mandate of the U.N. peacekeeping mission in South Sudan, known as UNMISS, in July. In December, the council approved a plan to almost double the number of peacekeepers to 12,500 troops as the violence worsened but so far only half those 5,500 reinforcements have arrived.

"We are refining the mandate of UNMISS so we have also to face the fact that maybe we can't cooperate with this government anymore because atrocities are committed by both sides," Araud said. "I do think that we have to have some soul searching about what should the U.N. do in South Sudan." (Editing by Sandra Maler and Paul Tait)